DE 10 2004 044 244, published at a later date, discloses as the hydraulic unit a spring system, in particular for use in the lifting frame of a forklift. The unit has at least one hydraulic accumulator and at least one switching valve for producing a fluid-carrying connection by a control line between the hydraulic accumulator and a working cylinder which can actuate the lifting frame. Lifting and lowering lines actuate the working cylinder into a raised or lowered position, with the lifting line and the lowering line each discharging into a main branch and being parallel to one another. One connecting point upstream and downstream from the switching valve discharges into the control line. Valves are connected to the main branches, which valves have an opposite action direction or actuation direction for each main branch. With this design a type of hydraulic Graetz circuit is implemented. With this arrangement, it is possible with only a few installation components and especially only using one hydraulic accumulator to achieve active load damping or cushioning in the lifting frame of the forklift. More energy-efficient operation is, however, not possible with this known solution.
The prior art furthermore discloses generic hydraulic units in which two working cylinders connected to one another on the piston side in the form of so-called plunger cylinders are connected to a conventional hydraulic pump. The hydraulic pump made as a constant delivery pump can have two opposite directions of rotation and accordingly two opposite transport directions for the fluid medium used. A control means, which can be actuated on the operator side as a control block, allows actuation of the hydraulic pump. A conventional electric motor is used for driving the pump. To extend the working cylinders to lift a load in the known solution, the hydraulic pump takes fluid from a tank storage vessel and pushes the removed fluid to the piston side of the respective working cylinder unit. To lower the load and thus to retract the working cylinders, the direction of rotation of the hydraulic pump is reversed. The fluid displaced via the piston side travels in turn to the tank side of the hydraulic unit. To raise and lower loads, the known unit requires high hydraulic pump work capacity and therefore a high driving power for the upstream electric motor. The known hydraulic unit is also limited with respect to the possible displacement speed for the working cylinders.